Here, radiographic systems include diagnostic X-ray apparatuses, which are equipped with an X-ray generator for irradiating the subject with X-rays and acquire tomographic images of the subject, based on the X-rays that have passed through the subject. As an example of diagnostic X-ray apparatus, there is an X-ray CT system configured to radiate X-rays with an X-ray tube that revolves around the body axis of the subject while the subject placed on the top plate of the system is being moved in the rostrocaudal direction. The system acquires CT images of the subject on the basis of the X-rays that have passed through the subject (e.g., patent reference 1).
The X-ray tube, which has a cathode and an anode, is operated in accordance with image-capturing conditions that include the tube voltage and tube current to be applied between the anode and the cathode, as well as X-ray focal spot size. For activation, the X-ray tube is applied with a high voltage, and, as the tube current tends to fluctuate in its rising up, the electric current flowing through the filament of the tube (hereinafter referred to as “filament current”) is carefully controlled for stabilizing the tube current at a desired value in order to generate X-rays from the anode at a predetermined output. The filament current rising up and flowing through the cathode when the radiography is started up is referred to as “initial value”, and the filament current while the tube current is stable is referred to as “stable value”. It is desirable to apply an appropriate initial value for achieving stabilization of the tube current within a very short time.
It is known that the filament of the X-ray tube undergoes aging and that, through the aging, the tube current becomes smaller for the same filament current. Because of this reason, even though the original initial value is appropriate, it becomes not appropriate after use for a long period of time, resulting in that an increasingly longer time is required for the tube current to stabilize.
For stabilizing the tube current in a very short time even after the aging of the filament, the initial value is adjusted for each of the image-capturing conditions by a service engineer when the X-ray CT system is installed or thereafter when it is tested periodically (these occasions are both hereinafter referred to as “at the time of adjustment”).
In a conventional method of adjusting the initial value, after X-ray generation is executed under predetermined image-capturing conditions, the output profile obtained is analyzed, and then the initial value of the filament current is modified and adjusted until X-ray outputs settle within a predetermined threshold value.
The initial value is adjusted for each of the points of the tube current predetermined with an increment of, for example, 10 [mA]. If some of the initial values are not actually adjusted, then they are calculated by linear interpolation with the initial values that have been adjusted for other points. The initial values actually adjusted for their corresponding points and the interpolated initial values are stored together in a table format in a storage. The stored initial values are used at the time of a next adjustment. Incidentally, the initial values that are used at the time of a next adjustment may be referred to as “next initial values”.
Tables are prepared for combinations of various tube voltages and focal spot sizes applicable. In other words, a table is prepared for each possible setting of the image-capturing conditions. If the X-ray CT system enables four tube voltage settings and two focal spot sizes as image-capturing conditions, then it will be equipped with eight such tables.
Furthermore, in recent years, there has been a tendency that X-ray CT systems are configured to speed up the revolving motion of the X-ray tube for the purpose of broadening their scanning range without increasing the amount of radiation the subject is exposed to. Accordingly, the X-ray generation is directed for an increasingly higher power for prevention of tube current shortage that can be otherwise caused by the increased higher revolving speed of the X-ray tube.